The visible imagery animation above shows stratocumulus over Wisconsin behind a strong early-season cold front. Careful examination of the animation will reveal the presence of at least three exhaust plumes from power plants over Wisconsin. Imagery from 1715 UTC, below, shows visible (0.63 µm) and infrared (3.9 µm and 10.7 µm) data (Click here for an image toggle without the Big Red Box). The plume is warmer in the 3.9 µm imagery, relative to its surroundings; the plume is cooler in the 10.7 µm imagery, relative to its surroundings (an enhanced version of the loop makes this even more evident). Why does the temperature difference exist?

Plumes appear darker — warmer — in the 3.9 µm imagery because of increasedreflectivity in the plume: cloud droplets in the power plant plume are smaller and more reflective of 3.9 µm radiation than the cloud droplets in the surrounding stratocumulus field. The plume is cooler in the 10.7 µm imagery because the plume is higher in the atmosphere than the surrounding stratocumulus deck.

Suomi NPP overflew the area at 1836 UTC, and that imagery is shown below. The higher resolution data allows a better discrimination of the small plumes over the state. As with GOES data, the shortwave infrared (3.74 µm for VIIRS) data also shows warmer conditions over the plume compared to the surrounding stratocumulus deck.

McIDAS images of GOES-15 0.63 µm visible channel data (above; click image to play animation) showed the hazy signature of a plume of re-suspended volcanic ash originating from the region of the Novarupta volcano in Alaska, moving southeastward over the Shelikof Strait toward Kodiak Island on 29 September 2014. The 1912 eruption of Novarupta left a very deep deposit of volcanic ash, which often gets lofted by strong winds in the early Autumn months before snowfall covers the ash (another example occurred on 22 September 2013). Surface winds gusted as high as 30 knots at regional reporting stations, with numerical models estimating terrain-enhanced winds as high as 40-50 knots over the Novarupta ash field.

An AWIPS II image of POES AVHRR 0.86 µm visible channel data (below) showed the ash plume at 22:46 UTC; a pilot report at 22:45 UTC indicated that the top of the ash plume was between 4000 and 6000 feet above ground level.

A sequence of 3 Suomi NPP VIIRS true-color Red/Green/Blue (RGB) images from the SSEC RealEarth web map server (below) indicated that the re-suspended ash plume had been increasing in areal extent during that period.

Tropical Invest 97L formed near Bermuda during the pre-dawn hours on 29 September 2014. After sunrise, AWIPS II images of GOES-13 0.63 µm visible channel data (above; click image to play animation) revealed a well-defined low-level circulation spinning just to the west of Bermuda. It is interesting to note that at 12:55 UTC a waterspout was reported 4 km to the east of the Bermuda International Airport (station identifier TXKF), associated with a band of deep convection that was moving northward (below).

GOES-13 0.63 µm visible channel image with Bermuda METAR observation

An overpass of a Metop satellite at 14:38 UTC provided a good view of the surface wind field with data from the ASCAT scatterometer instrument (below). There was one wind vector with a speed around 30 knots (green) just to the east of the center of circulation.

A comparison of Terra MODIS 0.65 µm visible channel and 11.0 µm IR channel images at 15:25 UTC (below) showed that the coldest cloud-top IR brightness temperatures of -55º C (orange color enhancement) were located to the north of the circulation center.

Previous posts on this blog (and elsewhere) have detailed the co-registration misalignment that exists between the 3.9 µm and 10.7 µm channels on the GOES-13 Imager. Because of this diurnally-varying co-registration error, a 3.9 µm pixel may be offset to the right or left of a 10.7 µm pixel; if this occurs near a pronounced temperature gradient (such as along a lakeshore), a false brightness temperature difference signal can ensue.

Consider, for example, the toggle above from 26 September 2014. A strong brightness temperature difference exists at 1825 UTC along the shorelines of Lakes Michigan, Huron and Erie; it is gone five minutes later, at 1830 UTC. There is no discernible change in the visible image over the same 5-minute interval (Link).

NESDIS operations alters the GVAR signal just before 1830 UTC (when the 3.9 µm imagery is shifted one pixel to the West) and at 0630 UTC (when the 3.9 µm imagery is shifted one pixel to the East) to mitigate the effects of the diurnally-varying co-registrations differences between the 3.9 µm and 10.7 µm channels. The imagery above shows the visible and two infrared (10.7 µm and 3.9 µm) channels at 1825 and 1830 UTC (GOES-13 was in Rapid Scan Operations mode at this time). The 3.9 µm imagery shows a one-pixel westward shift that is especially evident if you look at the unchanging navigation along the eastern shore of Lake Michigan. (1825 UTC imagery: Visible, 3.9µm and 10.7µm; 1830 UTC imagery: Visible, 3.9µm and 10.7µm) A similar link between 1815 and 1830 UTC on 25 September shows the same shift in the shortwave IR. A toggle between 0615 and 0630 UTC on 29 September shows the eastward shift in the 3.9 µm imagery that occurs then.